1. Field of the Invention
The present invention relates to a novel mesomorphic compound which may be used in a liquid crystal composition as well as liquid crystal devices apparatus and display methods using the same. In particular, the present invention relates to a novel mesomorphic compound and a liquid crystal composition, etc. with improved responsiveness to an electric field.
2. Related Background Art
Previously, liquid crystal materials have been used in electro-optical devices in various applications. Many such liquid crystal devices have used twisted nematic (TN) type liquid crystals, such as those shown in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich "Applied Physics Letters" Vol. 18, No. 4 (Feb. 15, 1971) pp. 127-128.
These devices utilize the dielectric effect wherein dielectric anisotropy causes in which the aligment of the average molecular axis of a liquid crystal to be directed in a specific direction in response to an applied electric field. However, the accepted lower limit of response speed of TN devices is on the order of milli-seconds, which is considered to be too slow for many uses. Moreover, while the simple matrix driving system (wherein scanning electrodes and signal electrodes are arranged in a matrix which is used by a multiplex driving scheme in which an address signal is sequentially, periodically and selectively applied to the scanning electrodes and prescribed data signals are selectively applied in parallel to the signal electrodes in synchronism therewith) is most promising for large-area flat displays in terms of cost, productivity, etc., the electric field applied to TN liquid crystal regions with a matrix driving system where a scanning electrode is selected and signal electrodes are not selected, or where a scanning electrode is not selected and a signal electrode is selected ("half-selected points") may cause image defects. In particular, if the difference between the voltage applied to the selected points and the voltage applied to the half-selected points is sufficiently large, and the voltage threshold (the voltage level required for allowing liquid crystal molecules to be aligned or be oriented perpendicular to an electric field) is set to a value therebetween, the TN display devices operate normally.
However, as the number (N) of scanning lines increases, a time (duty ratio) during which an effective electric field is applied to one selected point when a whole image area (corresponding to one frame) is scanned decreases with a ratio of 1/N. Accordingly, as the number of scanning lines becomes greater, the difference of the voltage values applied to a selected point and non-selected points when scanning is repeated decreases. As a result, this leads to drawbacks including lowering of image contrast or interference (crosstalk). These phenomena are regarded as essentially unavoidable when repeatedly driving liquid crystal materials which are not bistable (i.e. those materials wherein liquid crystal molecules are stably oriented horizontally with respect to the electrode surface and oriented vertically with respect to the electrode surface only when an electric field is applied) using a time storage effect.
In an attempt to overcome the above drawbacks, driving methods such as the voltage averaging method [Fundamentals and Applications of Liquid Crystals, Ohm. April 1979], the two-frequency driving method [Latest technology of Liquid Crystals. Kogyochosaka; September 1984], the multiple matrix method [Liquid Crystals. Applications, Baifukan. July 1985], etc. have been proposed. However, no method is sufficient actually to overcome these drawbacks. As a result, development of TN devices with large image areas (or high display element packaging densities) is delayed because it is difficult to sufficiently increase the number of scanning lines.
To overcome drawbacks with such prior art liquid crystal devices, the use of bistable liquid crystal devices has been proposed by Clark and Lagerwall (e.g. Japanese Laid-Open Patent Appln. No. 56-107216, U.S. Pat. No. 4,367,924, etc.). In this instance, bistable, ferroelectric liquid crystals having chiral smectic C-phase (SmC*) or H-phase (SmH*) are generally used which assumed different states with respect to the polarity of an applied electric field. Accordingly, these bistable liquid crystal materials and devices differ from optical modulation devices which use TN liquid crystals since the bistable liquid crystal molecules are oriented to first and second optically stable states respectively by one and the other electric field vectors. Moreover, the bistable liquid crystal has the further difference in that the two stable states which are assumed in response to an applied electric field are retained even in the absence of that electric field.
In addition to their bistability characteristic, ferroelectric liquid crystal ("FLC") materials also exhibit crystal an excellent high-speed responsiveness. This is because the spontaneous polarization of the ferroelectric liquid crystal and an applied electric field directly interact with each other to induce transition of orientation states. The resulting response speed is faster by 3 to 4 magnitudes than a response speed which is solely due to the interaction between dielectric anisotropy and an electric field.
Thus, a ferroelectric liquid crystal has excellent characteristics, and by making use of these properties, it is potentially possible to provide essential improvements to many of the above-mentioned problems concerning conventional TN-type devices. Particularly, the successful application of an FLC to a high-speed optical shutter, a high density display and a large picture display are all expected. For this reason, extensive research has been undertaken with respect to liquid crystal materials showing ferroelectricity.
However, ferroelectric liquid crystal materials developed heretofore cannot be said to satisfy optimal characteristics required for a liquid crystal devices including low-temperature operation characteristic, high-speed responsiveness, etc. Concerning deficiencies in response time .tau., the following relationship exists: .tau.=.eta./(Ps.multidot.E), where E is an applied voltage, Ps is the magnitude of spontaneous polarization and .eta. is viscosity. Accordingly, a high response speed can be obtained by any of (a) increasing the spontaneous polarization (b) lowering the viscosity, or (c) increasing the applied voltage. In practice, however, the driving voltage has a certain upper limit in view of driving with IC, etc., and should desirably be as low as possible. Accordingly, it is really feasible only to lower the viscosity or increase the spontaneous polarization.
A ferroelectric chiral smectic liquid crystal having a large spontaneous polarization generally provides in a cell a large internal electric field due to the spontaneous polarization. This internal electric field is liable to pose many constraints on the construction of a bistable device. [Ferroelectronics. 1988, Vol. 85. pp. 255-264]. Further, an excessively large spontaneous polarization often accompanies an increase in viscosity, so that a noticeable increase in response speed may not be achieved. Moreover, since response speed changes by a factor of about 20 over a typical operation temperature range of 5-40.degree. C., response speed variance actually exceeds the range which is controllable by manipulating driving voltage and frequency.
As described hereinabove, an optimal ferroelectric liquid crystal device therefore requires the availability of a chiral smectic phase liquid crystal composition which has a large spontaneous polarization, a low viscosity, a high-speed responsiveness and a response speed which depends minimally upon temperature.